In an operating room, ICU, or the like, it is sometimes necessary to continuously monitor the blood pressure of a patient. As a conventional technique of noninvasively and continuously monitoring the blood pressure, blood pressure estimation based on the pulse wave propagation time is known.
This technique uses the fact that the time (pulse wave propagation time) required for a pulse wave to propagate between two points in a living body or the pulse wave propagation velocity obtained by dividing the blood vessel length between the two points by the pulse wave propagation time has a correlation with the blood pressure. For example, the pulse wave propagation time is continuously measured and applied to an expression having a precalibrated coefficient, thereby continuously calculating and monitoring an estimated blood pressure (e.g., Japanese Patent Laid-Open No. 10-66681).
To measure the pulse wave propagation time, however, pulse waves must be measured in different locations, so the measurement requires a long time. Also, it is sometimes difficult to attach sensors or cuffs for measuring pulse waves to two locations. As described in Japanese Patent Laid-Open No. 10-66681, therefore, a general approach is to calculate the pulse wave propagation time by using an electrocardiogram (ECG) normally measured by a biological information monitoring apparatus and a pulse wave measured in one predetermined location (e.g., a fingertip) of a living body.
Unfortunately, the use of an ECG in the calculation of the pulse wave propagation time has a problem of the measurement accuracy. That is, an ECG is a signal which represents not a pulse wave but the electrical state change of the heart. There is a time difference (preejection period) between the timing at which the electrical state change occurs and the timing at which the heart actually contracts to generate a pulse wave. Accordingly, the pulse wave propagation time calculated by using the observation timing of the feature point of an ECG as a starting point contains an error caused by the preejection period.
If the preelection period is constant, this error is easy to correct. However, the preejection period changes from one person to another, and can change occasionally even in the same person. Therefore, an improvement of the accuracy by correction is limited.
Blood pressure monitoring apparatus normally performs control such that if blood pressure continuously measured on the basis of the pulse wave propagation time is abnormal, more accurate blood pressure measurement is performed by using a cuff or the like, and an alarm is output if an abnormal value is detected by this measurement.
Blood pressure measurement using a cuff is established as a method of noninvasively measuring the blood pressure, and effective to automatically obtain a well reliable blood pressure. However, this method requires avascularization, so the frequent use of the method is undesirable because the load on a patient increases. Therefore, accurate determination of the need for cuff blood pressure measurement is important not only to perform an appropriate therapy but also to reduce the load on a patient.
To increase the determination accuracy as described above, it is also important to increase the accuracy of the estimated blood pressure based on the pulse wave propagation time calculated from an ECG and a pulse wave observed at one point.